Method and device for the production of reticular structures

ABSTRACT

A method of producing reticular structures, particularly metallic reticular structures, as well as a device suitable for the production thereof. The method and device enable continuous and/or automated production of such structures, and particularly, large-scale automated production of large-dimensioned reticular structures. A reticulated foam pre-structure is placed into a first container and infiltrated with a refractory material. After solidification, the mold formed by the refractory material is removed from the first container and the foam pre-structure stripped from the mold. The mold is then pre-heated and placed into a second container and infiltrated with a molten substance that forms the reticular structure when solidified. The filled mold may be covered with a solid jacket as a means of controlling the rate and progression of solidification of the molten substance to form a fine-grained, bubble-free structure.

This application claims priority under 35 U.S.C. § 119 of the parentapplication DE 199 39 155.6, filed on Aug. 20, 1999 and PCT applicationPCT/DE00/02597, filed on Aug. 4, 2000.

BACKGROUND OF THE INVENTION

This invention relates to a method of producing reticular structures,and particularly, to the production of metallic reticular structures, aswell as to a device suitable therefor.

Reticular structures made from metal and other materials have a widerange of application. For example, these structures can be used aslightweight structural components, battery plates, electrochemicalanodes and cathodes, filters for fluids, separation devices for fluidmedia, heat shields, and for numerous other applications.

Numerous methods for producing such types of structures are known.Automated production of such reticulated structures, however, isextremely difficult to implement, primarily because, with theconventional methods, the reticulated foam bodies that serve as patternsor pre-structures must be bonded to wax plates. The step of bonding afoam pre-structure to a wax plate is almost impossible to automate. Thebonding points are, however, indispensable, since it is through thesepoints that the foam pattern is burned out and then, through theresulting junctions that the molten metal flows into the cavities orvoids formed by the foam pre-structure.

U.S. Pat. No. 3,616,841 (Walz: issued 1971) which is viewed as theclosest prior art, discloses a method for the production of an insolublefoam material with a predetermined reticulated structure. This methodencompasses the steps of producing a self-supporting reticulatedpolyurethane foam; producing a refractory mold material by filling thevoids of the polyurethane foam with a watery gypsum plaster suspensionthat then sets; heating the refractory mold material to a temperature ofabout 120° C. (250° F.) over a time period of two hours; producing voidsin the refractory mold material by raising the temperature of therefractory mold material to between 535 and 815° C. (between 1,000 and1,500° F.), in order to completely vaporize the foam and produce a mold;introducing a molten substance into the refractory mold in an amountsufficient to fill the voids which had been previously occupied by thereticulated foam pre-structure; solidifying the molten substance byreducing the temperature to below the melting point of the substance;and washing out the material that constitutes the refractory moldmaterial. The molten substance comprises metals, metal alloys, ceramicsand/or cermet.

The method disclosed by Walz has several disadvantages. The equipmentrequired for melting the substance that is poured into the refractorymold is either very expensive, especially for melting high-melting-pointmetals, or is technically not feasible. Another disadvantage is that inan automated process it is very difficult to control the bonding of thefoam to the wax plate. This step is critical, however, for controllingthe quality of the final product, as the quality of the bonding betweenfoam and wax plate determines the structure of the foam pre-structure,which, in turn, determines the technical parameters such as surfacesmoothness or dimensional accuracy, of the end product. Thus, in orderto reliably obtain an end product that corresponds to specification withregard to surface smoothness or dimensional accuracy or otherparameters, it is imperative that this step be controllable in order torestrict the statistical range of fluctuation in the structure of thefoam as much as possible.

To form the reticular structure, molten metal is poured into therefractory mold, which consists of branched voids. With the Walz method,in order to ensure that the molten metal remains liquid long enough toflow through the branches and completely fill the voids, the moldmaterial must be heated to a temperature higher than that of the meltingpoint of the molten metal. As a result, the solidification of the moltenmetal progresses very slowly, resulting in a solidified metal with acoarse grainy texture and reduced strength properties.

To solve this problem, Walz suggests various cooling methods, such as,for example, spraying the mold with water or air. A problem with suchcooling methods is that the mold hinders the flow of heat, therebysignificantly diminishing the cooling effect. Moreover, the productionof massive or solid areas of metal together with the reticular structureis related to the problem of a very slow cooling progress. In order toobtain a bubble-free and fine-grained texture, it is imperative that thesolidification process of the reticular structure be a controlledprocess. The method steps disclosed in Walz do not provide a means foreffective control over the solidification process. The Walz method hasan inherent economic disadvantage that limits the success or feasibilityof automating production processes for reticular structures, in that theslow progression of the solidification of the metal results in longprocess times.

What is needed, therefore, is an automated method of production ofreticular structures, particularly, metallic reticular structures. Whatis further needed, is such a method that produces a reticular structurehaving a fine-grained and bubble-free texture. What is yet furtherneeded is such a method that allows large-scale production of metallicreticular structures, including large-dimensioned reticular structures.

SUMMARY OF THE INVENTION

For the reasons cited above, it is an objective of the present inventionto simplify the production of a reticular structure so as to enableautomated production. It is a further objective to enable large-scaleproduction of reticular structures, including metallic and/orlarge-dimensioned reticular structures. It is a yet further objective toenable production of reticular structures having a fine-grained,bubble-free texture.

The objectives are achieved according to the present invention byproviding an automated method of producing reticular structures,including large-scale production of metallic and/or large-dimensionedreticular structures and by providing a device for the production ofsame.

With the method of the present invention, a foam pattern orpre-structure is used to create a refractory mold. The foampre-structure is placed in a refractory container and infiltrated with arefractory mold material, typically a gypsum plaster suspension. Afterthe mold material has solidified, the resulting mold, includingpre-structure, is withdrawn from the refractory container and thepre-structure removed from the mold by volatilization. The mold is thenpreheated to a temperature greater than the melting point of the moltensubstance that will form the reticular structure and placed inside aheat-resistant container. The molten substance for metallic reticularstructures may comprise metals, alloys, ceramics, cermet materials,and/or any suitable combination thereof.

A key feature of the method and device according to the presentinvention is that the heat-resistant container is greater in size thanthe size of the pre-heated mold when it is filled with the moltensubstance. After the mold is placed in the heat-resistant container andfilled with the molten substance, a solid jacket or shell is then pouredover the filled mold, filling a gap between the filled mold and the wallof the container. The container wall is temperature-controlled andmaintained at a temperature that is lower than the melting point of themolten substance. Since the jacket is in direct contact with thecontainer wall, heat is drawn from the mold through the jacket into thewall and, as a result, cooling begins at the outer perimeter andprogresses inward toward the center of the mold. After the moltensubstance has solidified, the mold is removed from the heat-resistantcontainer and stripped or removed from the cast reticular structure. Theability to control the temperature of the heat-resistant container andof the refractory mold promotes bubble-free solidification of the moltenmetal.

The method according to the invention offers several advantages. It isno longer necessary to bond the foam pre-structure to the running systemand sprue cup. This substantially reduces the time and material requiredto produce the casting mold. Because large areas of the foampre-structure are no longer bonded to the running system, the methodalso eliminates an inherent source of error that resulted from theuncontrollable method of bonding the pre-structure to the runningsystem. The method according to the invention is also economical, asonly the amount of refractory material that is required to produce themold for the actual reticular structure is used, thus reducing to aminimum the amount of refractory material used in the production of thereticular structure.

The method according to the invention provides additional advantagesthat improve the quality assurance for the structures. For example,following withdrawal from the first container, the foam pre-structureprotrudes from the refractory mold. This simplifies and improves visualmonitoring as to whether, after the foam pre-structure is volatilized,the ligaments and cells formed from the pre-structure will besufficiently well set externally to ensure a complete casting of thereticular structure. Moreover, the accessibility to all sides of thefoam pre-structure promotes rapid, even heating of the refractory mold.Ready access to the ligaments and cells of the foam structure alsopromotes rapid volatilization of the foam pre-structure. After thepre-structure has been volatilized, it is also easier to monitor whetherthe ligaments provide sufficient means of access of the molten metal tothe internal structure, that is, to the “negative mold.”

The method according to the invention does not require the use of thewax plates to bond the foam pre-structure, as do conventional methods,and allows continuous automated, large-scale production of reticularstructures. Examples of suitable uses of the metallic reticularstructures obtained from the production method according to theinvention, include use as catalysts for EMC shielding and in batteries.For example, in the production of a reticular structure for use in acatalytic converter for the combustion stabilization of diesel fuel, therefractory mold is filled with a molten metal comprising a Zn/Cu alloy.Reticular structures produced according to the method of the inventionand made of aluminum and then coated with lead are used, for example, inbatteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the foam pre-structure that can also be coated.

FIG. 2 shows the openable container with cover, in which the foampre-structure is placed according to the method of the presentinvention.

FIG. 3 shows the openable container containing the foam pre-structure,whereby the openable container has been filled with the refractory moldmaterial .

FIG. 4 shows the refractory mold material with the foam pre-structure.

FIG. 5 shows a refractory mold and the cavities that remain when thefoam pre-structure has been removed from the set mold material.

FIG. 6 shows the device according to the present invention, including acooling plate.

FIG. 7 shows the device of FIG. 6 on the cooling plate, with therefractory mold placed in the device, the heat-resistant container,whereby the refractory mold is geometrically smaller than the device.

FIG. 8 shows the reticular structure, formed according to the method anddevice of the present invention, with a partial jacket casting abuttingthe reticular structure.

DETAILED DESCRIPTION OF THE INVENTION

To form a reticular structure 22, shown in FIG. 8, according to themethod of the present invention, a reticulated foam pre-structure 10 isplaced in an openable container 12 having a container lid 12A, asillustrated by FIGS. 1 and 2. Preferably, the material used for thepre-structure material 10 is polyurethane foam, although any materialthat provides a sufficient number of pores is suitable for use as thepre-structure material. The foam pre-structure 10 is then infiltratedwith a refractory mold material 14, as shown in FIG. 3. The containerlid 12A is closed for applying a vacuum to the openable container 12.The refractory mold material 14 is allowed to solidify to form arefractory mold 16. Preferably, the refractory mold material 14 is awatery gypsum plaster suspension.

The surface of the foam pre-structure 10 is modifiable, preferably byroughening or structuring the surface of the foam pre-structure 10 afterit has been placed in the openable container 12. Pneumatic or vacuumassistance may be used to force the refractory mold material 14 into thecontainer 12 to ensure that the material 14 completely encases thepre-structure 10.

After solidification, the refractory mold 16, along with the foampre-structure 10, shown in FIG. 4, is withdrawn from the openablecontainer 12, and the foam pre-structure 10 stripped or removed from therefractory mold 16. FIG. 5 shows the refractory mold 16, including thevoids 17 formed by the foam pre-structure 10.

FIG. 6 shows the device according to the present invention, which is aheat-resistant container 18 mounted on a cooling plate 20. Therefractory mold 16 is pre-heated and placed into the heat-resistantcontainer 18. As shown in FIG. 7, the heat-resistant container 18 isgeometrically larger than the mold 16. The difference in dimensionsbetween the heat-resistant container 18 and the mold 16 results in a gap19 between the mold 16 and the heat-resistant container 18. The mold 16is then infiltrated with a molten substance that fills the voids 17 inthe mold 16, thereby forming a reticular structure 22, as shown in FIG.8. Any suitable casting material may be used in the method according tothe present invention. For metallic reticular structures, the moltensubstance comprises preferably metals, alloys, ceramics, metal ceramics,and/or any suitable combination thereof. After the molten substance hassolidified, the reticular structure 22 is withdrawn from theheat-resistant container 18 and the refractory mold 16 removed from thestructure 22.

As can be seen in FIG. 8, the reticular structure 22 corresponds inshape to the foam pre-structure 10. Also shown in FIG. 8 is a plate 24that is formed when a casting material is poured over the mold 16 thatis filled with the molten substance and fills the gap 19 between theheat-resistant container 18 and the mold 16.

The heat-resistant container 18 according to the invention holds themold 16 and has at least one opening 21 for pouring the molten metalinto the refractory mold 16. Preferably, the interior space of thecontainer 18 is larger than the pre-heated refractory mold 16 filledwith the molten substance, in order to provide a gap between a containerwall 18A of the container and the refractory mold 16. The size of thegap is freely-selectable and is determined by the difference in sizebetween the heat-resistant container 18 and the filled, pre-heatedrefractory mold 16. After pouring the molten substance into the mold 16,a solid jacket or shell is then cast onto the structure, i.e., therefractory mold 16 filled with the molten substance, thereby filling thegap 19 between the structure and the container 18. The container 18 istemperature-controlled and maintained at a temperature that is coolerthan that of the molten metal and the pre-heated refractory mold 16.Since the jacket is in direct contact with the container 18, heat isdrawn from the casting metal directly into the container 18 during thesolidification process, allowing the structure 22 to cool from theoutside inward toward the center of the refractory mold 16, therebyproducing a cast structure with a fine grain and, also, producingoptimal bonding between ligaments 22A of the reticular structure 22 andthe solid shell. The reticular structure 22 that is obtained aftersolidification of the molten substance can then be cleaned and ismodifiable, for example, by applying a conventional coating to thestructure 22.

The reticular structures 22 produced by the method according to theinvention, including the use of the heat-resistant container 18, can beintegrated into castings that are produced by various casting methods,such as, for example, die casting, permanent-mold casting, centrifugalcasting, low-pressure casting or back-pressure casting. The reticularstructures themselves can also be cast by these methods.

The method according to the invention enables automated production ofreticular structures 22 of the most varying degrees of fineness withrespect to the thickness of ligaments 22A and the size of cells 22B.Combinations of various cell sizes and ligament thicknesses within onestructure 22 are also possible.

The method and device according to the invention described herein aremerely illustrative of the present invention. It should be understoodthat variations in the steps of the method and construction of thedevice may be contemplated in view of the following claims withoutstraying from the intended scope and field of the invention hereindisclosed.

1. A method of producing a reticular structure, said method comprisingthe steps of: a) placing a reticulated foam pre-structure into a firstcontainer having a lid; b) infiltrating said foam pre-structure with arefractory mold material; c) solidifying said refractory material toform a refractory mold; d) withdrawing said refractory mold along withsaid foam pre-structure from said first container; e) removing said foampre-structure from said refractory mold; f) placing said refractory moldinto a second container having at least one opening for pouring in amolten substance and at least one wall, wherein said second container isgreater in size than said refractory mold so as to leave a gap betweensaid refractory mold and said at least one wall; g) infiltrating saidrefractory mold with said molten substance to form a reticularstructure; h) pouring a jacket material over said refractory mold so asto fill said gap with said jacket material to form a plate; i) coolingsaid reticular structure by maintaining a temperature of said secondcontainer, and particularly of said at least one wall, that is lowerthan a temperature of said molten substance so as to draw heat from saidreticular structure into said second container, and particularly throughsaid plate; and j) after said molten substance has solidified,withdrawing said reticular structure from said second container andremoving said refractory mold from said reticular structure.
 2. Themethod of producing a reticular structure according to claim 1, wherein,after said step of withdrawing said refractory mold from said firstcontainer, said foam pre-structure protrudes from said refractory mold.3. The method of producing a reticular structure according to claim 1,wherein said foam pre-structure has a surface and wherein, subsequent tostep a), said method includes a step of modifying said surface byroughening.
 4. The method of producing a reticular structure accordingto claim 1, wherein said foam pre-structure has a surface and wherein,subsequent to step a), said method includes a step of modifying saidsurface by texturing.
 5. The method of producing a reticular structureaccording to claim 1, wherein said reticular structure has a structuresurface and wherein, subsequent to step h), said method includes a stepof modifying said structure surface by applying a coating to saidstructure surface.
 6. The method of claim 1, wherein said reticularstructure is a metallic reticular structure and said molten substance isa molten metallic substance comprising materials from a group consistingof metals, metal alloys, ceramics and cermet.
 7. The method of claim 1,wherein said foam pre-structure is a polyurethane foam.
 8. The method ofclaim 1, wherein said refractory mold material comprises a gypsumplaster suspension.
 9. The method of claim 1, wherein said firstcontainer is made of a refractory material.
 10. The method of claim 1,wherein said second container is made of a heat-resistant material.